The invention relates to a device for multiplexing laser beams into a common output beam. An important application of this kind of multiplexing is for coupling together beams from a plurality of multimode laser diodes (LDs).
FIG. 1 shows parameters of a typical multimode LD 1. The LD 1 has a stripe 2 of length 100 μm, and width 2 μm. The direction along the stripe is often called the slow axis and the direction across the stripe the fast axis. In the figure, the fast axis coincides with the Y-axis and the slow axis coincides with the X-axis. The output beam from the LD 1 is characterised by waist sizes Wx and Wy. The output beam diverges from the near field to the far field. The divergence is highly asymmetric, resulting in a correspondingly asymmetric spatial distribution of optical radiation in the far field, illustrated schematically in the figure by the far-field beam cross-section 3. The divergence across the fast axis is characterised by an angle βx˜40° and is almost diffraction limited. The divergence in the Y-direction is characterised by an angle βy˜10°-14°. To characterise spatial distribution of an optical beam it is convenient to introduce the following parameter:M2=π(W/2)sin(β/2)/λ  (1)which defines the degree of dissimilarity of a beam under consideration from a diffraction-limited beam. From Eq. (1) one can find that for typical laser diode M2x=18 and M2y=1.2. In other words, output radiation from a laser diode is almost diffraction limited in the Y-direction (along the fast axis) and 18 times “worse” than diffraction limited beam in the X-direction.
The fact that output beam from LDs is essentially diffraction-limited in one direction and non-diffraction limited in the orthogonal direction allows for efficient spatial multiplexing when a number of beams are stacked in such a way that M2 is approximately equal in both directions. For example, if one stacks 15 beams along the Y-axis then such a source will have M2x˜M2y=18 (since 15×1.2=18). In applications where a single mode beam is not required, for example coupling light into a circular multimode fibre, the coupling efficiency is determined by M2max so that the coupling efficiency from a single stripe LD with M2x=18 and M2y=1.2 is approximately the same as from a LD stack with M2x=M2y=18, but the coupled optical power will be 10 times greater in the latter case.
This multiplexing or coupling principle was first used in 1968, as described in U.S. Pat. No. 3,396,344 [1] in which beam combining is achieved by collimating the fast axis and focusing by a cylindrical lens. More recent multiplexing designs are disclosed in U.S. Pat No. 5,319,528 [2] and U.S. Pat. No. 6,075,912 [3].